The present invention relates to a diagnostic method for an electric power steering systems, and to a system incorporating such a method especially during start-up.
It is well known to use an electric motor to apply an assistance torque to a part of a steering apparatus of a vehicle which is of the same sense as the torque applied by the driver, thus reducing the amount of torque needed to turn the wheel. The torque demanded by the driver is usually measured using a torque sensor which is connected to the steering column, and the output of the sensor which is indicative of the torque demand is used as an input for a motor controller. The controller produces a torque signal which is fed to a motor drive circuit, typically a set of motor phase switches that, by opening and closing, can selectively connect each phase of the motor to the positive or negative side of a battery. The motor drive circuit, in response to the output of the controller, ensures that that required current is applied to each phase at the right time to produce the desired torque from the motor.
It is also known to provide one or more relays in series between the motor phases and the drive circuit, or at the star or phase neutral point or even between the drive circuit and the battery. In a 3 phase arrangement then preferably 2 relays would be required either between the drive stage and motor phases or at the star or neutral point. When the system is shut down the relay contacts are open so that no current can flow through the motor phases. On start-up the relay contacts are closed to allow current to flow through the relay and the motor phases. They remain closed during use of the steering system until it is shut down or a fault is detected which could lead to a closed loop path through two phases the motor. By opening the relay contacts the loop is broken and current is prevented from following through the motor.
The applicant has appreciated that a potential problem may arise where dry relay contacts are used and the relays are left open for extended periods in harsh environments. It is known that under such conditions a film of dirt and corrosion can build up on the contacts which is insulating and may prevent the relay, upon closing, providing a flow path for the motor phase currents.
Relay wetting is known as a solution to the problem of film build up. Typically a high current, equal to the wetting current specified for the relay, is applied to the relay immediately that it is closed and held for a defined period of time. This current burns through the film and conditions the relay contact. The time required varies from relay to relay but typically a current of at least 10 Amps held for at least 0.5 seconds is required for a relay suitable for use in an electric power steering apparatus. A lower current of, say 5 Amps, could be used but would need to be applied for a longer time to wet the contacts. The applicant has appreciated that this traditional solution of relay wetting is not acceptable in a power steering application as it increases the delay between start-up and the system being available for use. This delay may be noticeable to a user of the system. Also, it has been appreciated that reducing the time by increasing the current is not acceptable is it would lead to a high torque being applied to the motor which might be felt through the steering wheel. Again this would not be acceptable.